Modern imaging radar sensors that are air- and space-borne use the principle of “radar with synthetic aperture” (SAR) for imaging static scenes. Furthermore, the imaging of moving objects—e.g. vehicles—with the inverse SAR process (ISAR) is increasingly becoming the focus of requirements of radar systems used for terrestrial observation and military reconnaissance. While the movement of the vehicle appears as interference with the relative movement between the radar sensor and the vehicle, with a suitable arrangement it can improve the imaging performance. In the context of a system design, an ISAR operational mode can therefore be sought that uses the movement of the vehicle to enhance performance. On this basis, the duration of an ISAR exposure can be significantly reduced in comparison with the prior art if the course of the vehicle to be imaged undergoes a continuous change in direction while it is being imaged, e.g. while traveling through a bend. This is particularly important for multi-functional monitoring radar that has to perform tasks in other modes before and after the ISAR data acquisition.
One problem of the evaluation of the ISAR data is that the necessary knowledge of the vehicle movement is generally not available or available only with insufficient precision for the image generation. The aspect angle course subject to which the radar illuminates the vehicle during the exposure and subject to which the vehicle echoes are received, plays a decisive role beside the changes in distance between the sensor and the vehicle, which can easily be estimated and compensated for. A change in the aspect angle at constant angular velocity then determines the scaling of the vehicle image perpendicularly to the radar's line of sight. A wrong assumption in this case leads to a distorted image. Furthermore, if the angular velocity is not constant, the imaging of the vehicle is out of focus, which is usually not usable.
The determination of the aspect angle course prior to the image generation therefore represents a significant problem, for which until now no satisfactory solution existed in the prior art.
It is already known from the general state of knowledge of the person skilled in the art to make the problem of the determination of the aspect angle course the responsibility of other operational modes in the case of multi-function radar. In the context of the system design, this requires that prior to performing the exposure moving vehicles are detected (MTI) and their courses are traced. Information regarding the aspect angle course during the ISAR exposure is then derived from the course of the vehicle.
This separation in time between the acquisition of the information regarding the vehicle movement and the acquisition of the ISAR radar echo inevitably gives rise to a problem if the vehicle behaves differently than expected. The predicted aspect angle course is in that case wrong and not usable. Admittedly this problem could be resolved through the operation of MTI, coursing control, and ISAR acquisition in the multiplex, but this represents a particular challenge that even the most sophisticated radar systems can satisfy only in exceptional cases.
P. Berens and J. H. G. Ender, “Motion Estimation for ISAR Imaging of Ground Moving Targets”, EUSAR 2006, Dresden, May 2006 proposed compressing the echoes in the context of the ISAR processing initially in the range direction, wherein the translational movement (change in distance between sensor and vehicle) is also compensated for. The entire duration of the exposure is then divided into short intervals. Corresponding to these time intervals the data are separated into so-called azimuth segments. Subsequently, the data of these segments in the azimuth direction are transformed into the Doppler frequency range, so that a sequence of distance Doppler segments is created. The reflectivity of the vehicle appears coarsely focused therein. The aspect angle course can subsequently be determined from the course of the reflectivity distribution orientation across all the segments.
It is, however, a disadvantage that the reflectivity distribution in the distance Doppler segments itself changes greatly with the aspect angle, which leads to errors in the estimation of the aspect angle course. Furthermore, the method itself requires an initial assumption regarding the velocity with which the aspect angle changes.
German patent DE 10 2006 009 121 A1 only discloses estimating two parameters for vehicles that travel at constant velocity, instead of determining the aspect angle course, and to facilitate the image generation with these. While the determination of the two required focusing parameters can take place in a very simple manner with this established method, it is unfortunately not usable for the determination of aspect angle progressions at non-constant angular velocity, as often occurs in practice.
Exemplary embodiments of the present invention determine for ISAR processing, from acquired ISAR data of a vehicle moving along a road and from information regarding the course of the road, the true aspect angle course, subject to which the radar illuminates the vehicle during the exposure and subject to which the vehicle echoes are reflected and received by the radar, with a method that is generally usable compared with the prior art.
According to exemplary embodiments of the present invention, the aspect angle course subject to which the radar illuminates a vehicle during the exposure and subject to which the vehicle reflects the echoes that are then received by the radar sensor is determined. Assuming the premise that the radar data alone are not sufficient for a robust determination of the aspect angle course, additional knowledge of the course of the road is used, subject to the assumption that the vehicle to be imaged is travelling on a road. Using the distance between the radar sensor and the vehicle as well as the change in distance, both of which are determined from the radar data, the position and the velocity of the vehicle are determined at any time during the exposure through the linkage with available road information. In so doing, one finds a location of the road whose distance from the sensor agrees with the measured distance of the vehicle as the vehicle position is obtained. Building further on the position, the vehicle velocity is then determined by projecting the velocity in the radar's line of sight onto the road direction at the determined vehicle position.
The aspect angle is then determined from the positions of the sensor and the vehicle as well as the direction of movement of the vehicle that corresponds to the direction of the vehicle's longitudinal axis.
Required road information can be acquired from digital maps or images (e.g. aerial photographs, SAR images).
As an advantage in comparison with previously known approaches, the aspect angle course is determined in the present invention from the actual acquired ISAR data. The otherwise unavoidable separation in time between the aspect angle determination and the ISAR data acquisition is thereby omitted and the determined aspect angle course reflects the actual geometric relations during the exposure. The aspect angle determination is advantageously no longer based on the orientation of the reflectivity distribution in the range-Doppler segments, but instead evaluates just the position of the radiometric centroid in distance and Doppler within the scope of the movement compensation, which takes place prior to the estimation of the actual aspect angle.
The applicability of the solution according to the invention is not limited to vehicles with a constant velocity.
The aspect angle course, subject to which the radar illuminates the vehicle, can thereby be determined for vehicles that travel on known roads or roads. Consideration of the method provides a sharp, undistorted image of the vehicle in the context of the generation of ISAR images. Furthermore, the road information can be used to take ISAR images specifically in areas in which vehicles are expected with motions that are suitable for ISAR.
The invention is explained in detail below with reference to the drawings.